The temperature control of the main spindle and the proper lubrication of the bearings, gears and the like within the main spindle head are indispensable in order to maintain the machining precision of a machine tool at a high precision in a stable condition.
FIG. 3 shows a conventional cooling oil circuit for a main spindle in a machining center.
Referring to FIG. 3, reference character 10 indicates the main spindle of a machine tool, and reference character 11 indicates a main spindle motor which drives the main spindle 10 in rotation. A cooling device 12 includes a storage tank 13 for storing lubricating oil, and a pump 14 which sucks and delivers the lubricating oil in the tank 13. The lubricating oil delivered from the pump 14 is cooled as it passes through a cooler 63. FIG. 4 shows the composition of a cooling circuit in which lubricating oil is cooled by the cooler 63. In this cooling device 12, the delivery side of a compressor 60 is connected to a condenser 61, a capillary tube 62 and the cooler 63 in this order, so that a refrigerating cycle is constituted.
The lubricating oil sucked by the pump 14 is thus fed to the cooler 63, in which heat exchange is performed. This cooled oil is delivered from an oil outlet 65 of the cooling device 12 and flows through the cooling oil circuit, i.e., flows regularly to an oil groove 16 arranged on the periphery of the main spindle 10 of the machine tool and to an oil Groove 17 provided round the main spindle motor 11, thereby absorbing heat from these parts, so that a rise in temperature of the main spindle motor 11 can be prevented.
In the meantime, an oil bath 18 is provided in the interior of the main spindle head. A lubricating oil circuit is provided in which the oil which has gathered in the oil bath 18 is sucked through an oil filter 19 by an oil pump 20, and the oil which has been delivered from the oil pump 20 is fed through a manifold 21, and oil feeding tubes 30, 31 and 32 to each part which requires lubrication, i.e., to the bearings 33a, 33b and 33c, gears 34 and the like within the main spindle head. The lubricating oil which has finished lubricating is adapted to be recovered into the oil bath 18 again.
In this way, the conventional machine tool is of such construction that the cooling oil circuit for the main spindle and the lubricating oil circuit for the main spindle head are independently provided, and even when the oil which has performed the lubrication of the main spindle head is returned from the oil pan to the oil bath, it is not cooled. From this reason, notwithstanding that the main spindle is cooled, the interior of the main spindle head, in which the bearings and gears as the heat-generating places are incorporated, incurs a rise in temperature in proportion to time along with a lapse in the operating hours of the machine tool.
This generation of heat causes a temperature gradient from the main spindle head to the main spindle, and heat transfer occurs at each portion of the machine tool. Thus, there is a problem in that such heat transfer continues until the distribution of temperature at each portion of the machine tool becomes constant, so, the machining precision comes to be unstable during such a period of time. There is a further problem in that as the heated lubricating oil is returned to the oil pan, a rise in the air pressure within the main spindle head is brought about, causing the lubricating oil to blow off from the lower portion of the main spindle head.